1. Field of the Invention
The present invention relates to transmitters for mobile communication systems, in which methods such as W-CDMA (Wideband-Code Division Multiple Access) and/or OFDM (Orthogonal Frequency Division Multiplexing), for example, are/is used, and in particular relates to a transmitter that detects and suppresses the peak power of a multi-carrier transmission signal.
2. Description of the Background Art
FIG. 16 shows an example of an internal configuration of a peak power suppression means 301.
As shown in Eq. (1), for example, a power calculation means 313 calculates, from I-phase (In-phase) and Q-phase (Quadrature-phase) components of an input signal, an instantaneous power value for each sample.Instantaneous Power=(I-Phase Component)2+(Q-Phase Component)2  Eq. (1)
A peak power detection means 314 compares a power value of an input signal with a threshold power for each sample to determine, as a peak power, the sample having a power value greater than the threshold power. As a result of this comparison, when the peak power is determined, the peak power detection means 314 outputs the power value of the peak power, and when no peak power is determined, the peak power detection means 314 outputs 0 data. If the threshold power is set at a low level, the peak power might occur continuously for several samples; therefore, when only the maximum of the continuous peak powers is regarded as an object to be suppressed, excessive suppression can be prevented.
It should be noted that in this example, the peak is detected using power value, but the peak is equally detected even if the square root thereof is taken and amplitude value is used.
A peak power suppression rate calculation means 315 determines the ratio between the peak power and the threshold power, and calculates a rate at which the peak power is suppressed to the level of the threshold power. In this example, the peak power is suppressed by subtracting an amplitude component exceeding the threshold value from a transmission signal, and therefore, the peak power suppression rate is calculated as shown in Eq. (2).
                              Peak          ⁢                                          ⁢          Power          ⁢                                          ⁢          Suppression          ⁢                                          ⁢          Rate                =                  1          -                                                    Threshold                ⁢                                                                  ⁢                Power                                            Peak                ⁢                                                                                          ⁢                                                                                        ⁢                Power                                                                        Eq        .                                  ⁢                  (          2          )                    
A delay adjustment means 312 supplies the IQ components of the input signal with delays equivalent to process delays, which occur at the power calculation means 313, the peak power detection means 314 and the peak power suppression rate calculation means 315.
Multipliers 321, 322 constituting a multiplication means 316 multiply the delay-adjusted IQ components of the input signal at the time of peak detection by the peak power suppression rate, and generate a peak power suppression signal that is a suppression amplitude component of the peak power.
A filter coefficient generation means 318 generates, based on carrier frequency information of the transmission signal, a filter coefficient having a frequency characteristic for limiting the frequency band of the peak power suppression signal to a desired frequency band. The frequency band of the peak power suppression signal is preferably similar to that of the transmission signal or preferably falls within the frequency band of the transmission signal in terms of the quality of spectrum waveform. It should be noted that the filter coefficient generated in this example is normally in the form of a complex coefficient in order to cope with any carrier frequency.
A complex multiplication means 317 complex-multiplies the peak power suppression signal by the filter coefficient, and outputs the peak power suppression signal whose band has been limited to a desired frequency band. In the case of using a W-CDMA method, for example, this complex multiplication is performed as shown in Eq. (3).I′=I×Coef—Re−Q×Coef—Im Q′=Q×Coef—Re+I×Coef—Im  Eq. (3)                ※I: I-phase component of Multiplication means 316 output signal        Q: Q-phase component of Multiplication means 316 output signal        I′: I-phase of complexmultiplication means 317 output signal        Q′: Q-phase of complexmultiplication means 317 output signal        Coef_Re: real part of filter coefficient        Coef_Im: imaginary part of filter coefficient        
As methods for band-limiting the peak power suppression signal in the complex multiplication means 317, two circuit implementation methods will be described below.
In a first method, an FIR filter is used. Since the filter coefficient is a complex coefficient, a filter operation is performed by convolution of complex multiplication.
Next, a second method will be described.
By limiting the peak power, serving as an object to be suppressed, to the maximum of continuous peak powers or to the maximum of peak powers during a certain sample interval in order to prevent excessive suppression as described above, the degradation in signal quality of the transmission signal after peak power suppression is reduced. In this case, the peak power suppression signal will be one in which a single impulse occurs during a certain interval.
FIG. 17 shows an example of time waveform in the case where samples, in each of which instantaneous power exceeds a set threshold value, are all detected as peak powers. The horizontal axis represents sample, while the vertical axis represents peak suppression signal power value.
FIG. 18 shows an example of time waveform in the case where a peak power, which is the maximum power during an interval of 50 samples, is extracted as a peak power serving as an object to be suppressed. The horizontal axis represents sample, while the vertical axis represents peak suppression signal power value.
Thus, even if no FIR filter is configured for a peak power suppression signal in which a peak power serving as the single maximum power during a certain interval is extracted as shown in FIG. 19, a peak suppression amplitude component is extended to a sample length equivalent to a table width (equal to the number of taps), in which a filter coefficient is stored, and is complex-multiplied by the filter coefficient for each sample, thereby obtaining a peak power suppression signal whose band is limited so as to be equivalent to that obtained when it is passed through an FIR filter.
In an FIR filter, due to a convolution operation, multipliers are required in accordance with the number of taps, but in the second band limitation implementing method shown in FIG. 19, the number of multipliers to be used is only four, which is required for complex multiplication, thus effectively preventing an increase in circuit size.
A delay adjustment means 311 supplies the IQ components of the input signal with delays equivalent to process delays, which occur along a path leading from the power calculation means 313 to the complex multiplication means 317.
Subtracters 323, 324, constituting a subtraction means 319, subtract the peak power suppression signal from the transmission signal for each of I phase and Q phase, and outputs the transmission signal whose peak power has been suppressed.
Hereinafter, an exemplary procedure of filter coefficient generation in the filter coefficient generation means 318 will be described in detail.
In a peak power suppression means according to an embodiment of the present invention described later, it is assumed that peak power suppression is performed on an IF (Intermediate Frequency) signal produced by multi-carrier synthesis, the frequency band of this input signal varies depending on the number of carriers and carrier frequency, and a filter coefficient for controlling the frequency band of the peak power suppression signal has to be changed to an optimum one in accordance with the frequency band of the transmission signal.
A filter coefficient associated with an IF signal can be generated by complex-multiplying the filter coefficient for a base band 1 carrier signal (in the case of a W-CDMA signal, a pass band width is 5 MHz, and a center frequency of the pass band is 0 MHz) by the carrier frequency. Next, the procedure of generating a filter coefficient associated with arbitrary carrier setting from a filter coefficient for the base band 1 carrier signal will be described.
The filter coefficient associated with the base band 1 carrier signal is normally not a complex coefficient but is a real coefficient. This is because the carrier frequency is 0 MHz which is a special case, and since the phase is not rotated, the phase is fixed at 0 degree, thus allowing the imaginary part of the filter coefficient to be 0. The filter coefficient “tap [k]” for the base band 1 carrier signal is defined by Eq. (4). In this equation, the number of taps for the filter is indicated by “L”, which is odd number.{tap[k]|−(L−1)/2≦k≦+(L−1)/2}  Eq. (4)
The pass band of the filter having the filter coefficient defined by Eq. (4) is frequency-changed by f1 (=ω1/2π) [MHz]. The complex filter coefficient after the frequency change is defined by Eq. (5).{(tapRe2[k],tapIm1[k])|−(L−1)/2≦k≦+(L−1)/2}  Eq. (5)
The filter coefficient, which has been frequency-changed by f1 (=ω1/2π) [MHz], is determined by Eq. (6) and Eq. (7).TapRe1[k]=tap[k]×cos(ω1·t+θ) where −(L−1)/2≦k≦+(L−1)/2  Eq. (6)tapIm1[k]=tap[k]×sin(ω1·t+θ) where −(L−1)/2≦k≦+(L−1)/2  Eq. (7)
Since it is a digital region, time t fluctuates with a time width per sample. θ is a phase offset, and in order to suitably suppress the peak power, θ has to be determined so that the following equation: (ω1·t+θ=0) is established at a center position of the filter coefficient, i.e., at k=0.
Next, the procedure of generating a filter coefficient having a pass band for a plurality of carriers by multi-carrier transmission of two or more carriers will be described.
The filter coefficient for the filter with a frequency fn(=ωn/2π) is defined by Eq. (8).{(tapRen[k],tapImn[k])|−(L−1)/2≦k≦+(L−1)/2}  Eq. (8)
The filter coefficients obtained by synthesizing all the filters associated with frequencies f1, f2, . . . , fn are represented by Eq. (9) and Eq. (10).tapRe[k]=tapRe1[k]+tapRe2[k]+ . . . +tapRen[k] where −(L−1)/2≦k≦+(L−1)/2  Eq. (9)tapIm[k]=tapIm1[k]+tapIm2[k]+ . . . +tapImn[k] where −(L−1)/2≦k≦+(L−1)/2  Eq. (10)
In the case of synthesizing a plurality of filter coefficients, gain adjustment has to be carried out. For example, in the case of synthesizing filter coefficients for two carriers, the synthesized filter coefficient is multiplied by ½, and gain is kept constant regardless of the number of carriers.
With the above-described procedures, filter coefficients associated with any number of carriers and carrier frequency can be generated. However, if carrier setting for a transmission signal is limited to a finite pattern, filter coefficients associated with all the transmittable carrier settings may be stored in a memory in advance, and a filter coefficient may be selected in accordance with frequency information of the transmission signal.
Next, FIG. 23 shows, as an exemplary configuration of the filter coefficient generation means 318, an exemplary configuration of a filter coefficient generation means 318a. 
In this example, filter coefficient storage parts J1 to Jn associated with a plurality of, i.e., an n number of, carriers 1 to n, respectively, store filter coefficients having frequency characteristics associated with respective transmission carriers.
An addition part 331 adds I phase components of filter coefficients outputted from the n number of the filter coefficient storage parts J1 to Jn, and outputs the added result as the coefficient of the I phase.
An addition part 332 adds Q phase components of filter coefficients outputted from the n number of the filter coefficient storage parts J1 to Jn, and outputs the added result as the coefficient of the Q phase.
Next, two examples of problems that occur in a peak power suppression means according to a conventional technology will be described using results of calculator simulation with a W-CDMA signal.
These examples of problems can occur by multi-carrier transmission of two or more carriers. However, in the following description, for the sake of simplicity, a transmission signal is transmitted by two-carrier transmission, and when the level between carriers is set in an unbalanced manner, a carrier f1 and a carrier f2 are set so that the carrier f1 is always at a higher level. In the calculator simulation, the carrier frequency was set at f1: −2.5 [MHz], and f2: +2.5 [MHz].
(i) Problem Example 1
At the time of two-carrier transmission, the filter coefficient has a pass band for two carriers; however, even if a level difference exists between the carriers of a transmission signal, the peak power suppression signal is kept at a constant level within the pass band.
FIG. 20 shows examples of frequency spectra of a transmission signal (i.e., an input signal of the peak power suppression means 301) and a peak power suppression signal (i.e., an output signal of the complex multiplication means 317) when the level difference between the carrier f1 and the carrier f2 is set at 12 dB. The horizontal axis represents frequency [MHz], while the vertical axis represents level [dB].
It can be confirmed that in the transmission signal, a level difference exists between the carriers, but in the peak power suppression signal, no level difference exists between the carriers. In this case, since the level difference between the transmission signal and the peak power suppression signal is small in the carrier f2, the signal quality within the band of the carrier f2 is significantly degraded as compared with that within the band of the carrier f1.
For example, in the case of a W-CDMA signal, the signal quality within the band is measured by EVM (Error Vector Magnitude), and/or PCDE (Peak Code Domain Error), but until the level of transmission power Pmax−18 [dB] (Pmax: maximum transmission power), EVM and/or PCDE standard(s) must be satisfied. Therefore, if the level unbalance between the carriers is taken into consideration, the peak power cannot be suppressed to a low level in order to satisfy the signal quality standard within the transmission band.
A table in FIG. 22 shows a summary of EVM and PCDE characteristics with respect to output signals of the peak power suppression means 301 according to the conventional technology.
As signals serving as objects to be evaluated, there are provided signals of two patterns, characteristics of which are obtained when the carrier f1 and the carrier f2 are at an equal level, and when a level difference therebetween is 12 dB. In either signal pattern, level adjustment is performed so that the total transmission power is kept constant at Pmax at a preceding stage of the peak power suppression means 301, and furthermore, peak detection threshold values are set at an equal level, thus making the suppressed amount of the peak power equal.
According to the characteristics shown in the table of FIG. 22, when the two carriers are at an equal level, 3GPP standard (EVM: 12.5 [%], and PCDE: −33 [dB]) is satisfied with a sufficient margin, but when there is a level unbalance, the characteristic of the low-level carrier f2 is degraded more significantly than that of the carrier f1, and does not measure up to the standard.
(ii) Problem Example 2
At the time of two-carrier transmission, if the level of the carrier f2 is extremely low (for example, if the power of f2 is Pmax−50 dB) or if the carrier f2 had a burst interval for a certain period of time (it is to be noted that frequency information is maintained at transmission state even during burst period), a peak power suppression signal might be produced at a higher level than the carrier.
FIG. 21 shows exemplary frequency spectra of a transmission signal (i.e., an input signal of the peak power suppression means 301) and a peak suppression signal (i.e., an output signal of the complex multiplication means 317) when the level difference between the carrier-f1 and the carrier f2 is set at 50 dB. The horizontal axis represents frequency [MHz], while the vertical axis represents level [dB].
It can be confirmed that in the carrier f2, the peak power suppression signal is produced at a higher level than the transmission signal.
See Japanese Unexamined Patent Application Publication No. 2005-20505.
For example, execution of peak power suppression on a transmission signal in a transmitting amplifier is an important technique for reducing the ratio of the peak power of the transmission signal to average power (PAPR: Peak to Average Power Ratio) and for decreasing the back-off of a power amplifier to increase the power efficiency thereof. Furthermore, if the PAPR of an input signal of a power amplifier is low, a power amplifier with a low saturation level can accordingly be used, which leads to the cost reduction of the power amplifier.
In the methodology for limiting the band of a peak power suppression signal to a frequency band similar to that of the transmission signal as described above, the degradation in spectrum waveform is very small, and the signal quality measured by EVM, PCDE or the like is lower compared with other peak power suppression methods, thus making it possible to reduce the PAPR to a lower level while satisfying the standard such as 3GPP. However, if a level difference exists between carriers in multi-carrier transmission as mentioned above, there have been caused problems that signal quality is significantly degraded and distortion is found in a burst interval.
Thus, according the conventional technology, a peak power suppression signal has been generated for a multi-carrier signal regardless of a level difference between carriers. Therefore, in the case where a level difference exists in a transmission carrier signal or in the case of a burst signal in which a carrier during an interval is not transmitted, there have been caused problems that the level of the peak power suppression signal becomes high with respect to the carrier signal, and radio specification cannot be satisfied.